<!doctype html>
<style>
#c {
  background: rgba(0,0,0,0.2);
}
</style>
<canvas id='c' width='300' height='300'>
<script>
  // bug crbug.com/416174
  var ctx = document.getElementById("c").getContext("2d");
  ctx.clearRect(0,0,150,150);
</script>
